Titan, Saturn’s largest moon, still holds many mysteries in its thick atmosphere despite a decade of observations from NASA’s Cassini spacecraft. What spurs the growth of large chemical compounds high in the moon’s ionosphere? Why is the chemistry so efficient at creating unsaturated and aromatic hydrocarbons?

To better explain Titan’s variable upper atmosphere, Westlake et al. enlisted the Ion Beam Spectrometer (IBS) on Cassini’s Plasma Spectrometer (CAPS) to measure the composition and concentration of large hydrocarbon ions. The researchers made use of data from a continuous flyby of the instrument; only partial flybys were available before that due to the mechanical sweeping of the instrument aperture. The flyby revealed vast chemical complexity in ion structure.

The team used data analysis and numerical models to isolate the key drivers for large hydrocarbon ion growth high in Titan’s atmosphere. They that found ion-molecule reactions are responsible for the growth in Titan’s heavy ions. Their research also shows that the heavy ions are created from acetylene and ethylene, which are each atmospheric building blocks. In turn, these large ions become a significant source of the massive hydrocarbons seen across the moon, including the middle and lower atmosphere, as well as the surface.

The authors call for laboratory tests that seek to reproduce reactions between these large hydrocarbons, as well as nitrogen-­containing hydrocarbon ions, to help figure out exactly what’s happening in this alien environment. (Journal of Geophysical Research: Space Physics,doi:10.1002/2014JA020208, 2014)

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